How does the central nervous system process information from proprioceptive sense organs? How is this information integrated into behavior? These questions are basic to an understanding of proprioception and motor control but have not been resolved in any system. The functions of proprioceptive sense organs have classically been evaluated according to their reflex effects. Unfortunately, the reflex effects of most proprioceptors are not constant, but show considerable plasticity, that is, reflexes can change according to the specific behavior or state of an animal. The behavioral functions and neuronal mechanisms underlying reflex plasticity have not been defined due to the large number of sensory neurons and interneurons in vertebrate proprioceptive systems and the lack of understanding of the specific functions of proprioceptors in behavior. The proposed proprioceptive system, the femoral chordotonal organ of the locust hindleg, is amenable to studies to examine the functions of reflex plasticity and to define these mechanisms at a cellular level. The following specific objectives will be addressed by the proposed experiments: 1) To define the functions of reflex plasticity, the chordotonal organ will be selectively mechanically stimulated in freely moving animals to mimick changes in joint angle. The reflexes this receptor elicits will be characterized during a variety of behaviors and the specific ranges of joint angle in which they act will be determined. 2) To define the neuronal circuitry of the system, the proprioceptive interneurons mediating these reflexes will be identified and their modes of reflex action will be characterized. The specific responses of interneurons to afferent input will be examined and correlated with the parameters of reflexes determined in freely moving animals. 3) To examine the cellular mechanisms underlying reflex phasticity, the activities of these interneurons will be examined during changes in behavior that are known to be accompanied by changes in proprioceptive reflexes. These studies will determine the patterns of neuronal activity that occur when proprioceptive reflexes are modulated by the central nervous system. These parallel investigations will generate a useful model system for understanding the basic mechanisms underlying processing of proprioceptive input and its integration into behavior.